1. Field of the Invention
The invention relates to a hydraulic control system for an automatic transmission in an automobile, and particularly, to facilities for controlling a down shift operation in such a hydraulic control system.
2. Description of the Related Art
In general, driving states of a moving vehicle can be classified into various different states such as a power-on state in which an accelerator pedal is depressed commanding transmission of torque from an engine to vehicle wheels, a power-off state in which the accelerator pedal is released commanding cessation of transmission of torque from the engine to the vehicle wheels, and further, a high speed driving state and a low speed driving state. Shift control in an automatic transmission is based on the vehicle driving state. For example, a down shift during the power-on state, commonly called a kick down shift, is performed by further depressing the accelerator pedal (throttle opening is increased), while a down shift in a power-off state is performed during reduction of the vehicle speed after the accelerator pedal is released (throttle opening is decreased).
A hydraulic shift procedure generally includes the engagement of a frictional element with the simultaneous disengagement of another frictional element, for example, the clutch to clutch shift operation described in Japanese patent publication Laying-Open No. Hei 4-210158. In this procedure the presence of a power-on state or a power-off state is detected, and based on the detected result, a down shift is performed by control logic set for the respective state. In the event that the vehicle driving state is changed to the power-on state by depressing the accelerator pedal during a downshift procedure under power-off control, it is determined to continue the downshift using the power-off control logic or to continue the downshift using the power-on control logic based on the hydraulic pressure for the disengaging frictional element.
More specifically, detection of the throttle opening is used to determine the power-on state or the power-off state. If the hydraulic pressure on the disengaging frictional element (off pressure command) is zero when the change to the power-on state from the power-off state is detected, the control logic for the power-off state is discontinued and the control logic for the power-on state is used. If the hydraulic pressure on the disengaging frictional element is not zero (hydraulic pressure still remains), the control logic for the power-off state is continued with the hydraulic pressure on the engaging frictional element increasing until the down shift procedure ends.
In conventional hydraulic controls, control in the power-on state and control in the power-off state are performed with different respective control logic. Therefore, for example, when the accelerator pedal is released during the operation of a down shift in the power-on state, the down shift is performed with the control logic for the power-on state. The power-on state control logic performs the down shift by decreasing the hydraulic pressure on the disengaging frictional element so that the rotational speed of the input shaft speed is normally increased to a synchronized rotation speed after the shift. However when the driving state has changed from the power-on state to the power-off state, the rate of increase of the input shaft rotation speed is low because the accelerator pedal is released and motor torque is low. Even though the decrease in hydraulic pressure on the disengaging frictional element is made quickly in an attempt to compensate for the low rate of increase in input shaft rotation speed, the input shaft rotation speed is not increased to the required synchronizing rotation speed. Therefore, it is necessary to increase the input shaft rotation speed by increasing the hydraulic pressure on the engaging frictional element.
However, increase in the hydraulic pressure on engaging frictional element by the power-on state logic is generally delayed, that is, the hydraulic pressure on the engaging frictional element is maintained at a low pressure until the input rotation speed is increased to about the synchronized rotation speed so as to prevent a tie-up with the disengaging frictional element. Therefore, the hydraulic pressure on the engaging frictional element is increased at a relatively low rate. As a result, the time required for a down shift is excessively long.
In one type of automatic transmission, as shown in FIG. 24, the gear ratios can be manually shifted to produce a feeling of a manual shift transmission by structuring the shift lever 100 to be movable to operating positions calling for different transmission ratios such as a first ratio, a second ratio, a third ratio, and a fourth ratio while in the D range of the automatic transmission. In the normal D range of an automatic transmission, the excessive delay in the down shift caused by a change from the power-on to the power-off state (for example, when a kick down shift is interrupted by release of the accelerator pedal) can be prevented by changing control of the transmission to an up shift mode. But if the down shift is being performed in response to movement of the manual shift lever, it is impossible to change to the up shift mode because of the manual demand for a down shift such as a shift from fourth ratio to third ratio. Therefore, the delay in the down shift cannot be prevented by discontinuing the down shift operation.
In a down shift in an automatic transmission in which a one-way clutch operates, the engaging frictional element is connected in parallel with the one-way clutch in the power-off state for performing an engine brake operation after the down shift. In this structure during a manual down shift in the power off state, a time lag occurs during the down shift and it is difficult to achieve a proper feel of the down shift at all vehicle speeds.